These guidelines of investment casting tolerances apply as a general rule of thumb. An investment casting is meant to be an intricately detailed "near net-shape" part that offers the most value and reality. If the buyer and producer continue a completely communication, the design requirement and cost efficiencies will be achievable by processing capability.

Linear Tolerances

The general rule-of-thumb for linear tolerance of investment casting is normally expressed as ±.005" per linear inch, of length (± .127 mm per 25 mm). As with any such rule, there are always exceptions required by process capabilities. The table below can be useful for most applications. Optimistic tolerances can be made available for a few selected design characteristics such as holes and slots. One should bear in mind however that tighter tolerances sometimes require special procedures and/or secondary operations such as machining which can increase the cost of the casting and the need for longer production times.

U.S.-Inches

Range

Standard

Optimistic

Up to 1/2"

±.007

±.003

1/2"-1"

±.010

±.005

1"-2"

±.015

±.010

2"-3"

±.020

±.015

3"-4"

±.025

±.018

4"-5"

±.030

±.020

5"-6"

±.032

±.022

6"-7"

±.035

±.025

7"-8" *

±.038

±.028

METRIC-Millimeters

Range

Standard

Optimistic

Up to 12.7mm

±.18

±.08

12.7-25.4mm

±.25

±.13

25.4-50.8mm

±.38

±.25

50.8-76.2mm

±.51

±.38

76.2-101.6mm

±.64

±.46

101.6-127.0mm

±.76

±.51

127.0-152.4mm

±.81

±.56

152.4-177.8mm

±.89

±.64

1778.-203.2mm

±.96

±.71

Flatness

Flatness refers to single plane surface and is usually function of volumetric shrinkage:
This feature of casting is measured as the distance between two parallel planes, within which, the entire surface of the part must lie. Once again, the actual shape of the part will largely influence the cooling behavior of the wax pattern and the part as it solidifies; and this, in turn, tends to govern process capability. This requirement is frequently expressed in terms of 6 sq. inches of surface area.

U.S.-Inches

Part thickness

Dish per 6 sq.inches of surface

Up to .250"

Insignificant

.251" - .500"

.004 in. T. I. R.

.501" - 1.000"

.008 in. T. I. R.

1.001" - 2.000"

0.016 in. T.I.R.

METRIC - Millimeters

Part thickness

Dish per 42 sq.cm of surface

Up to 6.5 mm

Insignificant

6.5 - 13 mm

.102 mm T. I. R.

13 -25 mm

.201 mm T. I. R.

25 -50 mm

.406 mm T. I. R.

Straightness, Roundness and Positioning

Tolerances for axial straightness is commonly expressed as ±.005" per inch. (±.127 mm per 25 mm).

Roundness (solid bar or hollow tube shapes) is a function of normal shrinkage variations in the metal. Shrinkage variation increases with diameter and the tolerance required increases proportionately. Tubular section out-of-roundness may be altered mechanically per to wall thickness and ductility. A general rule of .005" per inch (.127 mm per 25 mm) may be applied.

U.S.-Inches

Diameter

Tolerances

Up to 1"

.006 in. T. I. R.

1" - 1 1/2"

.008 in. T. I. R.

1 1/2"-2"

.010 in. T. I. R.

2"-3"

0.15 in. T.I.R.

METRIC - Millimeters

Diameter

Tolerances

Up to 25 mm

.152 mm T. I. R.

25 - 37.5 mm

.203 mm T. I. R.

37.5 - 50 mm

.254 mm T. I. R.

50 - 75 mm

.381 mm T. I. R.

-@Tolerance of positioning (holes or bosses) is stated as±.005"per
-@inch. (±.127 mm per 25 mm) form any single reference point.

Concentricity

Concentricity of diameters in a shaft is also a function of straightness too. Centers will be concentric within .005" per inch. (.127 mm per 25 mm) of maximum separation.

Center of a bore I. D. to O. D. of a part will be concentric within .003" per 1/2" (.076 mm per 13 mm) of wall thickness. This assumes measurements are taken in the same plane and neglect of out-of-roughness tolerance. If the concentricity is being measured in separate planes, the prerequisite of the above paragraph must be included.

Angularity

As-cast tolerances of angles is mainly a function of the part design and their location in casting. They range from ±1/2 degree (considered minimum) for well supported positions to ±2 degree where existing distortion could be expected. Inclusion of gussets and ribs normally minimizes distortion and many sections can be mechanically straightened.

Blind Holes

These feature are generally cast provided the depth is equal to the width or diameter for small characteristics less than 3/8" (9.40 mm). As the size increases above 3/8" (9.40 mm), the depth can move incrementally to double the size of the opening.

Parallel Sections

Controlling relationship is between length and width of the element being checked. To establish a specific tolerance range for all combinations would be impossible, but a general tolerance of .010" TIR per inch (.254 mm per 25 mm) may be used.

Although wall thickness down to .020" (0.50 mm) may be achievable in a small area, the normal minimal wall thickness range lies between .040"(1 mm) and .080" (2 mm) depending on alloy and square inches of surface area.